The present invention relates to magnetic field sensors based on magnetoresistive material and, more particularly, to such sensors requiring low usage of electrical power.
There are many situations in which there is a need to measure a magnetic field. Among such situations are the measurement of position or proximity of a magnetized portion of a structure, the readout of stored magnetic information, the measurement of current flows without the need of a measuring device in the current flow path, etc.
Many of the magnetic effects in such situations are relatively small and therefore require a sensitive magnetic sensor. A magnetic sensor capable of sensing such small magnetic field perturbations, and which is economical to fabricate, is provided on the basis of the magnetoresistive effect. Such magnetoresistive material based magnetic sensors can be fabricated using monolithic integrated circuit fabrication techniques, and so can not only be made economically but also made quite small in size. The magnetoresistive material is provided as a thin film when fabricated using monolithic integrated circuit techniques.
A magnetoresistive material based magnetic sensor is arranged by providing a magnetoresistive material to be used as an electrical resistor. A current is passed therethrough, and the voltage there across will depend on the effective resistance of the material over the path in which the current flows. That resistance value will depend in turn on the state of the magnetization of the material. If the magnetization is parallel to the current flow, the material will exhibit a maximum resistance, and it will exhibit a minimum resistance for magnetization perpendicular to the current flow.
In the magnetoresistive material there will be an effective magnetization, and this will be directed primarily along the easy axis of the material. An external field acting on the magnetoresistive material will rotate the magnetization direction therein to change the resistance of that material as a result. The changed resistance carrying the current causes a voltage drop change across the resistor which can be sensed as an indication of the magnitude of the external field.
The effective resistance of such a film will vary as the square of the cosine of the angle between the effective magnetization direction and the current flow direction through the material. The total resistance, however, is usually not of interest but rather the change in resistance in response to a change in the applied external magnetic field. This change is often best measured at a point along the squared cosine response curve where the curve approximates a linear function.
To provide operation on such a linear portion of the response curve requires that there be an initial angle between the direction of current flow and the nominal direction of magnetization in the absence of any externally applied fields. This can be accomplished in alternative ways in a bias arrangement. The magnetoresistive material can be placed on the device substrate as a continuous resistor in a "herringbone" pattern or set of continuously connected multiple inclines, with the angle of incline being approximately 45.degree. with respect to the direction of extension of the resistor. There then must be provided a source for a magnetic bias field to be pointed in a direction which is 90.degree. to the direction of the extension of the resistor.
Another method is to provide a linear strip of magnetoresistive material, but to add individual conductors across that strip at an angle of 45.degree. with respect to the direction of the strip. This, in effect, causes the current to flow at an angle through the magnetoresistive strip with respect to the direction of elongation of the strip itself. This latter configuration is often called a "barber pole" sensor because of its configuration, and such an arrangement can eliminate the need for an external source of a magnetic bias field.
For low power usage, such a "barber pole"sensor is most effective if the magnetoresistive material strip is very long with respect to its width. This increases the resistance of the magnetoresistive material strip between its ends to thereby lower the power required to operate the device, and further, it substantially reduces demagnetization effects within the magnetoresistive material strip. However, there are limits to the length that such a strip can be continuously provided in a monolithic integrated circuit chip, and so the strip is often folded into a series of parallel portions. Each portion, as a series link, then folds back with respect to the preceding series link to thereby form a much more compact magnetoresistive material structure.
This leads to a relatively sharp curvature, however, at the locations where the magnetoresistive material in the folded strip completes one series link portion and then is bent around 180.degree. to continue in a direction parallel to the preceding link to form another. These sharp turn regions lead to the formation of so-called magnetic end-domains in and near these bend regions. The magnetization direction in such domains can vary substantially with respect to the direction of magnetization in the parallel portions of the continuous strip. The magnetization of the parallel portions themselves will be well aligned with the direction of extension of each portion because of the length compared to the width thereof leading to reduced demagnetization effects therein.
A particular difficulty with such end-domains is that they are susceptible to change with temperature and external magnetic field excursions, and so does the magnetization direction therein. This has the effect of causing random fluctuations in the resistance of the strip and so in the voltage value occurring across the magnetoresistive material sensor for a given current therethrough. If four such magnetoresistive material sensors are used in a bridge circuit, these voltage variations due to thermal changes in the end-domains will lead to randomly changing values or fluctuations in the voltage of the bridge between the sense connections leading to errors in measurements based on a magnetometer using such a bridge. Thus, there is a desire to provide a magnetoresistive magnetic sensor having a more stable voltage occurring across the magnetoresistors therein despite temperature excursions.